Comprises Wasco, Klamath, Lake, Sherman, Gilliam, Wheeler, Morrow, Grant, Union, Crook, Umatilla, Wallowa, Baker, Harney, and Malheur counties in the State of Oregon, U.S.A., an area of 65,683 square miles. It was established in 1903. The Rev. Charles J. O'Reilly, rector of the Church of the Immaculate Heart of Mary, Portland, Oregon, and editor of the "Catholic Sentinel" was named its first bishop and consecrated 25 August of that year. The diocese has a Catholic population estimated at about 4,000, whose spiritual needs are cared for by ten diocesan and seven Franciscan and Jesuit priests. The Sisters of St. Francis, St. Dominic, and the Most Holy Name of Jesus and Mary conduct five schools and academies. At the Umatilla Indian reservation there are more than 500 Catholic Indians attended by the Jesuit fathers of the Rocky Mountain Mission, two Brothers of Christian Instruction, and eight Sisters of St. Francis. There are 13 churches and 36 mission stations in the diocese.

Bishop O'Reilly was born 4 January, 1862, at St. John, New Brunswick, Canada, and educated at the Christian Brothers' school of St. John and at St. Joseph's College, Memramcook. He made his theological studies at the Grand Seminary, Montreal, and was ordained priest at Portland 29 June, 1890. He was then appointed to the mission of Oswego and Tegardville, and in February, 1894 was made rector of the Church of the Immaculate Heart of Mary, Portland.